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CHAPTER 20 Optoelectronics Objectives Describe and Analyze: • Cathode Ray Tubes (CRT) • Liquid Crystal Displays (LCD) • Light Sensors • Optoisolators • Lasers • Fiber Optics Introduction Optoelectronics includes: • Display devices, such as CRT or LCD computer screens. • Light sensors, including infrared security systems. • Digital data transmission over fiber-optic cables. A Tube? The original “active device”. What’s a Tube? • On February 18, 1908, Lee DeForest was granted U.S. Patent No. 879,532 for a vacuum tube triode he called the Audion. It was the first electronic device that could amplify a signal. – An incandescent filament (like in a light bulb) gets the cathode hot enough to boil electrons off it. – Positive voltage applied to the plate causes current to flow. – A small negative voltage applied to the grid (a metal screen) controls the flow of electrons. Cathode Ray Tubes (CRTs) Phosphors emit light when hit by electrons. CRTs Color CRTs have three guns, one for each color. CRTs • The electron beam in a CRT is deflected back and forth by the horizontal sweep circuit. • The beam is deflected up and down by the vertical deflection circuit. • Oscilloscope CRTs use electrostatic deflection. • Television CRTs use magnetic deflection. It is more efficient for large deflection angles. • CRTs are bulky and consume a lot of power but, so far, nothing has replaced them. Liquid Crystal Display (LCDs) Very low power, but relatively small and slow. LCDs Compared to CRTs: • Color LCD displays are expensive, and bigger ones are very expensive. • LCDs are not as bright. • LCDs cannot update the screen as fast. • LCD displays are thin and lightweight.